Indicating torque wrenches are known in the art, and include deflection beam and dial indicating wrenches. Recently, electronic techniques have been applied to improve the convenience and usefulness of conventional torque wrenches. The patent to Lehoczky et al., U.S. Pat. No. 4,125,016 discloses an electronic strain-gauge torque wrench which purports to provide increased accuracy by use of electronic means for sensing and displaying the applied torque. An analog signal representing the applied torque is produced by strain-sensitive foil-type resistors. This analog signal is converted into digital signals by an iterative process, and a difference or error signal is created for incrementing or decrementing a stored count until the difference signal is reduced to zero. The stored count then is a digital representation of the applied torque.
Since the analog signal is obtained via a resistive bridge network which includes the strain-sensitive resistive elements, offsets or inaccuracies are introduced due to nonlinearities in the resistors, temperature variations, and other error-introducing factors.
In particular, use of the Lehoczky device in one direction, such as clockwise for repeatedly tightening bolts, may introduce a permanent offset due to the fact that one of the strain sensitive resistors will have been repetitively placed in a compression mode while the other of the strain sensitive resistors will have been placed in an extensive mode. A permanent offset of this nature can only be corrected by rebalancing the resistive bridge. Consequently, after each application and release of torque, there is no assurance that the reading will be zero unless steps are taken to re-balance the bridge to compensate for any growing permanent offset.
Another problem with the Lehoczky torque wrench is calibrating the device to ensure that the displayed torque is an accurate representation of the applied torque. In the construction of torque wrenches, there are always some variations in the parameters of the elements which require compensation, such as modulus of elasticity of the handle or the characteristics of the electronic devices.
The Lehoczky device is calibrated by adjustment of a potentiometer which is apparently only usable for full scale calibration. The use of a potentiometer for calibration, together with the heavy reliance of analog circuit techniques, makes the Lehoczky device susceptible to thermal drift.